1. Field of the Invention
The present invention relates to an interferometric measuring method and an interferometric measuring apparatus applied to, e.g., an apparatus for aligning a mask with a wafer and an overlay accuracy measuring apparatus or a surface configuration measuring apparatus.
2. Related Background Art
FIG. 16 is a view illustrating the construction of a first conventional example. Shown therein is an alignment apparatus based on an optical heterodyne method using rectilinear diffraction gratings as alignment marks which are disclosed in Japanese Patent Laid-Open Application No. 64-82624. A diffraction grating 2 and a mirror 3 are arranged on the light path of a 2-wavelength rectilinear polarizing laser light source 1. An illumination optical system 4 is provided in the reflecting direction of the mirror 3. A mask 6 disposed on a mask stage 5 and a wafer 8 disposed on a wafer stage 7 are provided in the transmitting direction of the illumination optical system 4. Beams of light from the illumination optical system 4 are incident on a diffraction grating 9 formed on the mask 6 and a diffraction grating 10 formed on the wafer at an angle determined by the NA (Numerical Aperture) thereof.
Further, a polarized beam splitter 11 is provided in the reflecting direction of the diffraction gratings 9, 10. A knife edge 12 and a detecting means 13 are provided in the reflecting direction of the polarized beam splitter 11. The diffracted beam from the diffraction grating 9 is cut off by the knife edge 12 so that the light does not fall on the detecting means 13. A knife edge 14 and a detecting means 15 are provided in the transmitting direction of the polarized beam splitter 11. The diffracted beam from the diffraction grating 10 is cut off by the knife edge 14 so that the light does not fall on the detecting means 15. Further, outputs of the detecting means 13, 15 are connected to a phase-difference unit 16. An output of the phase-difference unit 16 is connected to a wafer stage driving circuit 17 and a mask stage driving circuit 18. An output of the wafer stage driving circuit 17 is also connected to the wafer stage 7. An output of the mask stage driving circuit 18 is connected to the mask stage 5.
A beam of light L1 emitted from the 2-wavelength rectilinear polarizing laser light source 1 is incident on the diffraction grating 2. Beams L2, L3, L4 from the diffraction grating 2 are deflected by the mirror 3. A 0th-order diffracted beam L3, a + first-order diffracted beam L2 and a - first-order diffracted beam L4 pass through the illumination optical system 4, and thereafter, any one of these diffracted beams is cut off. One of two other diffracted beams is changed in terms of its polarizing direction by means of a .lambda./2 plate, whereby the diffraction gratings 9, 10 are irradiated with this beam. A diffracted beam L5 is reflection-diffracted by the diffraction grating 10. A diffracted beam L6 is reflection-diffracted by the diffraction grating 9. The diffracted beams L5, L6 travel in the same direction. However, the positions of the diffraction gratings 10, 9 slightly deviate in the y-direction as shown in FIG. 17. Hence, the diffracted beams L5, L6 do not overlap but are slightly separated.
The diffracted beam L5 is split into two beams by the polarized beam splitter 11. The transmitted beam is, however, cut off by the knife edge 14 and is not therefore detected. Only the reflected beam is detected by the detecting means 13. Similarly, only the transmitted beam in the diffracted beam L6 is detected by the detecting means 15. The phase difference between the respective beat signals is detected by the phase-difference unit 16 from the outputs of the detecting means 13, 15.
FIGS. 18 and 19 are explanatory views showing the first conventional example where the mask 6 is aligned with the wafer 8. A synthesized beam Em of the - first-order diffracted beam having a frequency f1 and the + first-order diffracted beam having a frequency f2 is expressed by the following formula: EQU Em=A1.multidot.exp {i.multidot.(2.pi..multidot.f1.multidot.t-.delta.m)}+A2.multidot.exp {i.multidot.(2.pi..multidot.f.multidot.t+.delta.m)} (1)
where .delta.m is the x-directional displacement from the fiducial position in the diffraction grating 9 and is given by .delta.m=2.pi..multidot..DELTA.Xm/P. Note that P is the pitch of the diffraction gratings 9, 10.
The beam intensity Im of the synthesized beam Em expressed by the formula (1) is given by the following formula: EQU Im=.vertline.Em.vertline..sup.2 =A1.sup.2 +A2.sup.2 +2A1.multidot.A2.multidot.cos {2.pi..multidot.(f1-f2).multidot.t-2.delta.m(2)
The phase of the optical beat signal given by the formula (2) changes by a variation 4.pi..multidot..DELTA.Xm/P of a component expressed by the third term in the formula (2) which is produced when the diffraction grating 9 moves by .DELTA.Xm.
The value of the pitch P of the diffraction gratings 9, 10 is already known. It is therefore possible to detect the moving quantity .DELTA.Xm of the diffraction grating 9 by detecting the phase change of the optical beat signal.
Similarly, the moving quantity .DELTA.Xw of the diffraction grating 10 on the wafer 8 is also detectable. An optical beat signal Iw detected by the second detecting means 1 is defined as a beam intensity of the synthesized beams of the + first-order diffracted beam having the frequency f1 and the - first-order diffracted beam having the frequency f2. The optical beat signal Iw is expressed by the following formula: EQU Iw=A.sub.1.sup.2 +A.sub.2.sup.2 +2A1.multidot.A2.multidot.cos {2.pi..multidot.(f1-f2).multidot.t-2.delta.w} (3)
where .delta.w=2.pi..multidot..DELTA.Xw/P.
Further, the following is a formula to express the phase difference .DELTA..phi. between the optical signal detected by the detecting means 13 which is expressed by the formula (2) and the optical beat signal detected by the detecting means 15 which is expressed by the formula (3). EQU .DELTA..phi.=4.pi..multidot.(.DELTA.Xm-.DELTA.Xw)/P (4)
Detected in this way is the phase difference between the beat signal of the mask diffracted beam and the beat signal of the wafer diffracted beam. The mask stage 5 and the wafer stage 7 are moved relative to each other by the wafer stage driving circuit 17 and the mask stage driving circuit 18 so that the phase difference is zero. The mask is thereby precisely aligned with the wafer.
Further, the alignment performance of the assembled apparatus as an exposure apparatus is actually measured and evaluated. For this purpose, the hyperfine patterns formed on the mask have hitherto been overlay-printed on the wafer. The deviation quantity between the on-wafer patterns is measured. A well-known vernier measuring method is employed, as illustrated in, e.g., FIG. 18, so-so that called vernier patterns are exposure-formed on the wafer, and the deviation quantity therebetween is observed by enlarging the patterns through a microscope, etc..
That is, FIGS. 20, 21, and 22 are explanatory views each showing, e.g., resist patterns exposed on the wafer. The resist patterns exist on the portions indicated by oblique lines. Note that these figures show the means for measuring only a deviation in the x-axis direction. At this time, patterns 21, 22 constitute a vernier. One graduation of the vernier is equivalent to 0.055 .mu.m. To start with, an exposure on the wafer is performed through a second mask (reticle), thereby forming the pattern 21. Next, the resist is coated thereon, and the second mask (reticle) formed with the pattern 22 is aligned with the wafer. Thereafter, the exposure is effected to form the pattern 22.
After this formation, as a result of the alignment, there is measured a degree of error with which the first mask (reticle) and the second mask (reticle) are overlaid. This measurement involves such a step that, as shown in FIG. 20, the vernier patterns on the wafer on which both of the patterns 21, 22 are printed are observed by enlarging them through the microscope and are then read. For example, referring to FIGS. 20 to 22, a pitch of the patterns 21 may be set to 7.95 .mu.m, while a pitch of the patters 22 may be set to 8.00 .mu.m.
Further, FIG. 23 shows a third conventional example by showing a surface configuration measuring apparatus to which the optical heterodyne method is applied. A collimator lens 26, a polarized beam splitter 27, acoustic optical elements 28, 29, a beam Splitter 30 and a mirror 31 are sequentially arranged on the light path of a light source 25. Besides, a beam splitter 32, a polarized beam splitter 33, a .lambda./4 plate 34, an objective lens 35 and a sample table 36 are sequentially arranged in the reflecting direction of the polarized beam splitter 27. An object S to be measured is placed on the sample table 36. A polarized beam splitter 37 is further provided in the reflecting direction of the beam splitters 30, 32. A photoelectric detector 38 is provided on the light path for the two synthesized beams. A polarized beam splitter 39 is also provided in the reflecting direction of the polarized beam splitter 33. A polarizing plate 40 and a photoelectric detector 41 are arranged on the light path for the two synthesized beams.
Beams emitted from the light source 25 are split by the polarized beam splitter 27, depending on the difference in their of polarizing components. One beam is frequency-modulated by the acoustic optical elements 28, 29 and subsplit into two beams by the beam splitter 30. This reflected beam is synthesized by the polarized beam splitter 37 with the beam reflected by the polarized beam splitter 27 and the beam splitter 32. This is detected by the photoelectric detector 38, and a reference beat signal is generated.
On the other hand, the beam penetrating the beam splitter 32 travels through the polarized beam splitter 33 and the .lambda./4 plate 34 as well. The transmitted beam is condensed by the objective lens 35 and falls on the surface of the object S to be measured. The beam reflected by the surface again penetrates the objective lens 35. At this time, this beam penetrates the .lambda./4 plate 34 twice, whereby the polarizing direction is turned through 90 degrees. The beam is thus reflected by the polarized beam splitter 33. This beam is synthesized with the beam penetrating the beam splitter 30 by means of the polarized beam splitter 39. The synthesized beams are rearranged in terms of their polarizing direction by the polarizing plate 40 and interfered with each other. This interference signal is detected by the photoelectric detector 41, thereby obtaining a measurement beat signal. The rugged configuration on the surface of the object S to be measured is measured based on the phase difference between the reference beat signal and the measurement beat signal.